1. Field of the Invention
The present invention relates to a phase-change nonvolatile memory device and a manufacturing method thereof, and more particularly, to a phase-change nonvolatile memory device in which a phase-change material layer is formed of an Sb—Zn alloy and brought into contact with an electrode layer capable of emitting heat due to externally supplied current to enable a reversible change between crystalline and amorphous phases, and a method of manufacturing the phase-change nonvolatile memory device.
2. Discussion of Related Art
Semiconductor devices may be categorized as either volatile memory devices or nonvolatile memory devices. A dynamic random access memory device (DRAM), which is a typical volatile memory device, needs to be refreshed during operation.
A low-integrated memory device consumes only a small amount of power to perform a refresh operation, while a highly-integrated memory device consumes a very large amount of power. For example, when a current refresh rate of 1 to 10 ms/Mbit is applied, the DRAM consumes a considerable amount of power on the whole. Specifically, in a current 1-Gbit DRAM, power consumed to perform a refresh operation dominates almost the entire power consumption. Despite an increase in power consumption, the DRAM is the most widely used memory module because it operates at high speed and is inexpensive.
If a nonvolatile memory device replaces a DRAM, it can be expected that not only power consumption but also operation time will be greatly reduced. Therefore, a vast amount of research has been conducted on nonvolatile memory devices lately. Among nonvolatile memory devices, flash memory has been most highly developed and is most widely used. However, since a flash memory operates at low speed and requires a relatively high voltage, it is utilized only for digital cameras and portable phones.
Meanwhile, memory devices should be highly reliable in rewrite operations. Although the flash memory is not very reliable in rewrite operations, when it is put to limited use in mobile devices, the number of times data is rewritten can be set to a small number. However, the rewrite reliability of the flash memory, which can be guaranteed in mobile devices, may be insufficient for stable operation of general-use personal computers (PCs).
Also, in order to satisfy various recent requirements of memory devices, a method of combining a DRAM, a static random access memory (SRAM), and a flash memory in an appropriate manner is being adopted. However, this method leads to a large increase in the entire size of a memory chip and is quite costly. For this reason, an integrated memory device that can be stably mounted on various devices for a variety of purposes is absolutely necessary. The integrated memory device strongly needs to have nonvolatility, high-speed, low power consumption, and high rewrite reliability, but semiconductor memory devices having all these characteristics have not yet been put into practical use. Therefore, exhaustive research into various nonvolatile memory devices has been progressing recently in search of possibilities and practicability of the respective nonvolatile memory devices from many angles.
Meanwhile, a phase-change nonvolatile memory device, which is called a phase-change RAM (PRAM), employs a phase-change material of which resistance depends on a crystal state thereof. That is, current or voltage is applied under appropriate conditions to control the crystal state of phase-change material so that data is stored in the PRAM. Also, the kind of stored data is read due to a change in resistance relative to the crystal state of the phase-change material. In this process, the PRAM performs a memory operation.
The PRAM may use a conventional phase-change material, such as a chalcogenide metal alloy, which is commonly employed for optical data storage devices, such as rewritable compact discs (CD-RWs) or digital versatile discs (DVDs). Since the manufacturing process of the PRAM is highly compatible with that of conventional Si-based devices, the PRAM may be easily embodied to have an integration density equal to or higher than that of DRAMs. Currently, the application of a Ge—Sb—Te chalcogenide material to PRAMs is being considered. However, in order to put the PRAMs to practical use, it is necessary to reduce power consumed during operation of the PRAMs, increase operating speed of the PRAMs, and control malfunctions caused by crystallization of an amorphous material and phase separation even after repetitive use of the PRAMs.
Conventionally, a Ge—Sb—Te chalcogenide metal alloy, especially, Ge2Sb2Te5(GST) with a composition ratio of 2:2:5, is being widely used as the phase-change material. Since the GST with the foregoing composition ratio is generally used as an essential material for optical storage devices that use a change in phase due to laser beams, the physical properties of the GST are well known. Therefore, the GST may be easily applied to PRAMs, and thus most PRAMs are manufactured using GST at present.
Meanwhile, the integration density of semiconductor memory devices using GST as a phase-change material (hereinafter, GST memory devices) has reached about 256 Mb, and it is known that GST memory devices have very good operating characteristics (refer to S. J. Ahn et al., Tech. Dig. Symp. VLSI Tech. 2005, pp. 98-99). When doing research on advanced nonvolatile semiconductor memory devices, the integration density of about 256 Mb requires great improvements in process and device technology. By comparison, ferroelectric memory devices or magneto-resistance memory devices have an integration density of about 16 to 32 Mb due to difficulties in manufacturing and embodying the devices. Therefore, a PRAM has excellent scaling characteristics and is regarded as the most suitable memory device that can substitute for a conventional flash memory.
However, the PRAM should perform more stable operations in highly-integrated memory modules in order to take the place of flash memory. In particular, a phase-change material with better physical properties is needed to embody gigabit PRAMs. Since a conventional phase-change material (i.e., GST) has a high melting point of about 620° C., it is difficult to sufficiently cut down an operating current. Also, the GST is crystallized at a relatively low temperature of about 147° C., so that an amorphous recording layer is highly likely to be crystallized during operation. Further, in order to operate a PRAM as fast as a DRAM, it is also necessary to develop a phase-change material having fast phase-transition speed and, in particular, fast crystallization speed.
Therefore, when providing a method of manufacturing a PRAM using a new phase-change material that has a lower melting point than a conventional phase-change material (i.e., GST) and a faster crystallization speed than the GST, and is crystallized at a higher temperature than the GST, a high-speed low-power PRAM can be manufactured by a simple process at low cost.
In order to fulfill the above-described requirements of a high-quality PRAM, the following methods are predicted.
A first method is aimed at shortening a SET operation time taken to crystallize a phase-change material, which occupies the longest time during operation, using a rapidly crystallized material.
In a second method, the use of a phase-change material with a low melting point makes it easier to change the phase-change material into an amorphous phase, which consumes the largest current during operation. Therefore, the phase-change material with the low melting point can reduce the amount of current required to put the phase-change material into an amorphous phase during fusion and cooling processes.
In a third method, a phase-change material having a high crystallization temperature is employed. A phase-change material, which is put to a crystalline or amorphous phase, may be undesirably crystallized due to thermal crosstalking (i.e., heat emitted by an adjacent cell) during operation of a PRAM. In this case, the use of the phase-change material having a high crystallization temperature can prevent the thermal crosstalking.
In a fourth method, the composition of a phase-change material is simplified to prevent phase separation caused by repetitive phase transition. A conventional chalcogenide material, which is an N-added Ge—Sb—Te-based material, is being presently used for PRAMs with excellent characteristics. However, after repetitive drives of the PRAMs, the chalcogenide material may be separated into stable phases, such as GeTe and Sb2Te3, thereby causing malfunctions.
Therefore, the present inventors have done research on phase-change materials formed of various metal alloys and come to a conclusion that when an Sb—Zn alloy is used as a phase-change material, a SET operation time can be shortened and a PRAM can stably operate at high speed, thereby enhancing reliability of the PRAM and greatly reducing power consumption of the PRAM.